1. Field of the Invention
The present invention relates generally to an Orthogonal Frequency Division Multiplexing (OFDM) communication system, and in particular, to an apparatus and method for transmitting data in an OFDM communication system.
2. Description of the Related Art
OFDM and space-time coding have recently received a great deal of interest as fundamental technologies for supporting high data rates required for future-generation wireless communication service. OFDM is a transmission scheme in which one serial data stream is divided into Nc parallel data streams and simultaneously transmitted on Nc sub-carriers. Given a sufficient Nc value and a sufficient guard interval, each sub-channel experiences frequency flat fading, making it possible to use a modulation scheme with a relatively high modulation order. Due to the advantages of high bandwidth efficiency and robustness under a multi-path channel environment, OFDM was adopted as the standard of a wireless LAN (Local Area Network) system such as IEEE (Institute of Electrical and Electronics Engineers) 802.11a or ETSI (European Telecommunications Standards Institute) HIPERLAN (High PERformance LAN) type2, and a broadcasting system such as DAB (Digital Audio Broadcasting) or DVB-T (Digital Video Broadcasting-Terrestrial).
Space-time coding provides spatial diversity through a plurality of transmit antennas under a fading channel environment. The results of many studies on space-time trellis codes and space-time block codes under a frequency flat fading channel environment have recently been reported. Specifically, Alamouti's space-time block code offers a full diversity gain at a full rate in a system using two transmit antennas and a low decoding complexity. Therefore, it has been adopted as a standard for 3rd generation (3G) mobile communication systems, such as WCDMA (Wideband Code Division Multiple Access) and CDMA2000.
OFDM systems using space-time block coding and space-frequency block coding based on the Alamouti's code have been proposed in the recent years. Assuming that a channel has not changed over two successive OFDM symbol periods, the Alamouti's code can be applied to the two OFDM symbols. This is called Alamouti's-Space-Time Block Code-Orthogonal Frequency Division Multiplexing (A-STBC-OFDM). If a channel has not changed with respect to adjacent sub-carriers, the Alamouti's code can be applied to the adjacent sub-carriers. This is called Alamouti's-Space-Frequency Block Code-Orthogonal Frequency Division Multiplexing (A-SFBC-OFDM).
FIG. 1 is a block diagram of a conventional transmitter in an A-STBC-OFDM/A-SFBC-OFDM system using Alamouti's code. Referring to FIG. 1, in the conventional transmitter, a serial-to-parallel (S/P) converter 102 converts Nc information symbols received from a data source 100 to a symbol vector Ds of length Nc, as is shown below in Equation (1).
                              D          s                ⁢                  =          Δ                ⁢                  [                                                    D                s                            ⁡                              [                0                ]                                      ,                                          D                s                            ⁡                              [                1                ]                                      ,                                                  ⁢            …            ⁢                                                  ,                                          D                s                            ⁡                              [                                                      N                    c                                    -                  1                                ]                                              ]                                    (        1        )            
Nc is assumed to be equal to an IFFT (Inverse Discrete Fourier Transform) length. It is a power of 2.
Using two successive symbol vectors Ds and Ds+1, an A-STBC-OFDM coder 104 generates four space-time code symbol vectors X1,s, X2,s, X1,s+1 and X2,s+1 to be transmitted in sth and (s+1)th OFDM symbol periods. The space-time code symbol vector X1,s can be generalized as in Equation (2),
                              X                      l            ,            m                          ⁢                  =          Δ                ⁢                  [                                                    X                                  l                  ,                  m                                            ⁡                              [                0                ]                                      ,                                          X                                  l                  ,                  m                                            ⁡                              [                1                ]                                      ,                                                  ⁢            …            ⁢                                                  ,                                          X                                  l                  ,                  m                                            ⁡                              [                                                      N                    c                                    -                  1                                ]                                              ]                                    (        2        )            where 1=1, 2 and m=s, s+1. Xl,m[k] represents a space-time code symbol transmitted on a kth sub-carrier in an mth OFDM symbol period through an 1th transmit antenna.
Because the A-STBC-OFDM coder 104 is based on Alamouti's space-time block code, in Equation (3),
                              [                                                                                          X                                          1                      ,                      s                                                        ⁡                                      [                    k                    ]                                                                                                                    X                                          1                      ,                                              s                        +                        1                                                                              ⁡                                      [                    k                    ]                                                                                                                                            X                                          2                      ,                      s                                                        ⁡                                      [                    k                    ]                                                                                                                    X                                          2                      ,                                              s                        +                        1                                                                              ⁡                                      [                    k                    ]                                                                                ]                ⁢                  =          Δ                ⁢                  [                                                                                          D                    s                                    ⁡                                      [                    k                    ]                                                                                                                    D                                          s                      +                      1                                        *                                    ⁡                                      [                    k                    ]                                                                                                                                            D                                          s                      +                      1                                                        ⁡                                      [                    k                    ]                                                                                                -                                                            D                      s                      *                                        ⁡                                          [                      k                      ]                                                                                                    ]                                    (        3        )            where x* is the complex conjugate of x. Further, in Equations (4a), (4b), (4c), and (4d):X1,s=[Ds[0], Ds[1], . . . , Ds[Nc−1]]  (4a)X2,s=[Ds+1[0],Ds+1[1], . . . , Ds+1[Nc−1]]  (4b)X1,s+1=[D*s+1[0], D*s+1[1], . . . , D*s+1[Nc−1]]  (4c)X1,s=[−D*s[0],−D*s[1], . . . , −D*s[Nc−1]]  (4d)
Two IFFTs 106 and 108 inverse-discrete-Fourier-transform the space-time code symbol vectors Xl,m and outputs four signal vectors xl,m, as shown below in Equation (5):
                              x                      l            ,            m                          ⁢                  =          Δ                ⁢                  [                                                    x                                  l                  ,                  m                                            ⁡                              [                0                ]                                      ,                                          x                                  l                  ,                  m                                            ⁡                              [                1                ]                                      ,                                                  ⁢            …            ⁢                                                  ,                                          x                                  l                  ,                  m                                            ⁡                              [                                                      N                    c                                    -                  1                                ]                                              ]                                    (        5        )            where xl,m[n] is an nth sample of an OFDM modulation symbol to be transmitted in an mth OFDM symbol period through an 1th transmit antenna. xl,m[n] is expressed in Equation (6),
                                          x                          l              ,              m                                ⁡                      [            n            ]                          ⁢                  =          Δ                ⁢                              1                          N              c                                ⁢                                    ∑                              k                =                0                                                              N                  c                                -                1                                      ⁢                                                            X                                      l                    ,                    m                                                  ⁡                                  [                  k                  ]                                            ⁢                                                          ⁢                              W                N                                  -                  nk                                                                                        (        6        )            where n=0, 1, . . . , Nc−1 and
      W    Nc    m    ⁢      =    Δ    ⁢            ⅇ                        -          j                ⁢                              2            ⁢                                                  ⁢            π            ⁢                                                  ⁢            m                                N            c                                .  
Parallel-to-serial (P/S) converters 110 and 112 convert the samples xl,m[n] to serial data streams. CP (Cyclic Prefix) inserters 114 and 116 insert CPs into the serial data streams and transmit them through transmit antennas 118 and 120, respectively.
As described above, the A-STBC-OFDM transmitter performs four IFFT operations for two successive OFDM symbol periods and the IFFTs 106 and 108 are required for the individual transmit antennas 118 and 120.
Because an A-SFBC-OFDM transmitter is the same in structure as the A-STBC-OFDM transmitter, except for using an A-SFBC-OFDM coder rather than an A-STBC-OFDM coder, the A-SFBC-OFDM transmitter will be described herein below using FIG. 1. However, in this description, the A-STBC-OFDM coder 104 will be replace with an A-SFBC-OFDM coder 104.
As in the A-STBC-OFDM transmitter, in an A-SFBC-OFDM transmitter, the S/P converter 102 converts Nc information symbols received from the data source 100 to the symbol vector Ds of length Nc expressed in Equation (1).
Using the symbol vector Ds, an A-SFBC-OFDM coder 104 generates two space-frequency code symbol vectors X1,s and X2,s to be transmitted in the sth OFDM symbol period. The space-frequency code symbol vector X1,s is generalized in Equation (7),
                              X                      l            ,            s                          ⁢                  =          Δ                ⁢                  [                                                    X                                  l                  ,                  s                                            ⁡                              [                0                ]                                      ,                                          X                                  l                  ,                  s                                            ⁡                              [                1                ]                                      ,                                                  ⁢            …            ⁢                                                  ,                                          X                                  l                  ,                  s                                            ⁡                              [                                                      N                    c                                    -                  1                                ]                                              ]                                    (        7        )            where 1=1, 2 and Xl,s[k] represents a space-frequency code symbol transmitted on a kth sub-carrier in the sth OFDM symbol period through an 1th transmit antenna.
Because the A-SFBC-OFDM coder 104 is based on Alamouti's space-time block code, in Equation (8),
                              [                                                                                          X                                          1                      ,                      s                                                        ⁡                                      [                                          2                      ⁢                      v                                        ]                                                                                                                    X                                          1                      ,                      s                                                        ⁡                                      [                                                                  2                        ⁢                        v                                            +                      1                                        ]                                                                                                                                            X                                          2                      ,                      s                                                        ⁡                                      [                                          2                      ⁢                      v                                        ]                                                                                                                    X                                          2                      ,                      s                                                        ⁡                                      [                                                                  2                        ⁢                        v                                            +                      1                                        ]                                                                                ]                ⁢                  =          Δ                ⁢                  [                                                                                          D                    s                                    ⁡                                      [                                          2                      ⁢                      v                                        ]                                                                                                                    D                    s                                    ⁡                                      [                                                                  2                        ⁢                        v                                            +                      1                                        ]                                                                                                                        -                                                            D                      s                      *                                        ⁡                                          [                                                                        2                          ⁢                          v                                                +                        1                                            ]                                                                                                                                        D                    s                    *                                    ⁡                                      [                                          2                      ⁢                      v                                        ]                                                                                ]                                    (        8        )            where k=2v,2v+1,v=0,1, . . .
            N      c        2    -  1.Further, in Equations (9a) and (9b),X1,s=[Ds[0], Ds[1],. . . , Ds[Nc−2], Ds[Nc−1]]  (9a)X2,s=└−D*s*[1], D*s*[0], . . . , −D*s[Nc−1], D*s[Nc−2]┘  (9b)
The two IFFTs 106 and 108 inverse-discrete-Fourier-transform the space-frequency code symbol vectors Xl,s and outputs two signal vectors xl,s, as shown below in Equation (10):
                              x                      l            ,            s                          ⁢                  =          Δ                ⁢                  [                                                    x                                  l                  ,                  s                                            ⁡                              [                0                ]                                      ,                                          x                                  l                  ,                  s                                            ⁡                              [                1                ]                                      ,                                                  ⁢            …            ⁢                                                  ,                                          x                                  l                  ,                  s                                            ⁡                              [                                                      N                    c                                    -                  1                                ]                                              ]                                    (        10        )            where xl,s[n] is an nth sample of an OFDM modulation symbol to be transmitted in the sth OFDM symbol period through the 1th transmit antenna. xl,s[n] is expressed in Equation (11).
                                          x                          l              ,              s                                ⁡                      [            n            ]                          ⁢                  =          Δ                ⁢                              1                          N              c                                ⁢                                          ⁢                                    ∑                              k                =                0                                                              N                  c                                -                1                                      ⁢                                                            X                                      l                    ,                    s                                                  ⁡                                  [                  k                  ]                                            ⁢                                                          ⁢                              W                N                                  -                  nk                                                                                        (        11        )            
The P/S converters 110 and 112 convert the samples xl,s[n] to serial data streams. The CP inserters 114 and 116 insert CPs into the serial data streams and transmit them through the transmit antennas 118 and 120, respectively.
As described above, the A-SFBC-OFDM transmitter performs two IFFT operations for one OFDM symbol period and the IFFTs 106 and 108 are required for the individual transmit antennas 118 and 120.
FIG. 2 is a block diagram of a typical transmitter in a conventional A-STBC-OFDMIA-SFBC-OFDM system. It is noted from FIG. 2 that the number of IFFT operations increases in proportion of the number of transmit antennas.
In the above-described conventional A-STBC-OFDMI A-SFBC-OFDM transmitter, an IFFT operation is performed for each transmit antenna to generate a transmission signal. Therefore, computation complexity is high and power consumption is increased.
Aside from Alamouti's code-based OFDM systems, OFDM systems using space-time/space-frequency block coding based on space-time block codes require more transmit antennas perform IFFT operations in proportion to the number of transmit antennas. Consequently, the implementation complexity of transmitters is considerably increased. Therefore, there is a need for a method of reducing transmitter implementation complexity in an OFDM system based on space-time/space-frequency block coding.